BACKGROUND AND OBJECTIVES: It is known that sympathetic reinnervation, following a heart transplantation, increases with time. 123I metaiodobenzylguanidine (123I-MIBG) is taken up by myocardial sympathetic neurons in a manner similar to norepinephrine (NE), and is used as an imaging agent. The purpose of this study was to evaluate cardiac sympathetic reinnervation, following an orthotopic heart transplantation, using 123I-MIBG scintigraphy. SUBJECTS AND METHODS: Twenty four 123I-MIBG images of the chest were taken in 15 patients (10 males, 5 females, mean age: 35+/-13 years), 1 to 48 (mean 10.8+/-11.9) months after a transplantation. Two healthy adults were studied as normal controls. The 123I-MIBG images were acquired at 15 minutes, and 4 and 24 hours, after an intravenous injection of 185 MBq 123I-MIBG. To quantitate the degree of myocardial uptake of the MIBG, the heart to mediastinal ratio (HMR) was measured. In nine patients, the 123I-MIBG scintigraphy was repeated one year later. RESULTS: The HMRs of the 4 and 24 hour images (1.26+/-0.23, 1.06+/-0.10, respectively) were lower than those of the 15 minute images (1.48+/-0.28). Twelve subjects, 1 to 12 months after the transplantation, showed no visible myocardial activities, but 12 subjects, 13 to 48 (28.6+/-12.8) months after the transplantation, showed visible myocardial 123I-MIBG uptakes (HMR: 1.65+/-0.21). The HMRs were high in normal controls (mean 2.84). One-year follow up scintigraphy showed increased HMRs compared with those taken immediately postoperatively (1.40+/-0.31 to 1.61+/-0.16, p<0.05). CONCLUSION: Partial sympathetic late reinnervation can occur one year after a transplantation.